This invention relates generally to the manufacture of valve seats and more particularly to improvements in forming valve seats of the type which have plastic face seals for sealing against the valve member.
It has long been recognized that a plastic insert known as a face seal may be attached to a metallic valve seat to provide an effective seal against a gate or ball valve member. The inert is usually formed of material available under the trademark "Teflon", although various other substances having similar properties may be used with satisfactory results. Typical methods which have been used in the past to attach the face seals to the valve seats are disclosed in U.S. Pat. No. 3,087,232 to Dow and U.S. Pat. No. 3,222,762 to Nowlin. Although these methods have been used successfully for the most part, they have not been entirely satisfactory in all respects.
The primary problem has been that the plastic insert is not always bottomed out completely against the bottom of the groove in the metal seat ring. Due to distortion caused by hammering of the insert into the groove or by imperfections and deformities in the insert, the outer surface of the insert is often uneven. Consequently, when a power driven ram is applied to the insert to press it into the groove in a manner causing it to cold flow into the serrations of the groove, there are gaps between the bottom of the insert and the bottom of the groove. Such gaps also result when the bottom of the groove is uneven or when a non-uniform driving force is applied to the ram, as sometimes occurs. After the insert has been attached to the seat in this manner and machined to provide a sealing surface, the seat is installed in a seat pocket of the valve body.
In service, the fluid pressures that are applied to the valve act against the sealing surface of the insert and can press the insert further into the groove in those areas where gaps exist. This causes portions of the sealing surface of the insert to be depressed far enough that they cannot effectively seal against the valve member. In some cases, the sealing surface is actually depressed within the groove so that it cannot seal at all. As can easily be appreciated, the lack of a good seal results in substantial leakage of the valve.
Defective inserts are sometimes discovered before the valve is put in service, as during a shell test of the valve body wherein the valve is subjected to fluid pressure of approximately one and one half times working pressure to test the integrity of the valve body. However, even if the defective insert is discovered during the shell test, the valve must still be disassembled and the seats must be removed and provided with new inserts if they are to function effectively. In addition, the time and expense of machining the defective insert has already been incurred.
A somewhat similar problem exists with respect to seat inserts of the type that are held in place by lips of the seats which are swaged into the inserts. After swaging of the lip, the sealing surface of the insert is machined and the seat assembly is installed in the valve body. If the insert is completely bottomed in its groove prior to swaging of the lip, the fluid pressure applied to the valve during service can bottom out those portions of the insert where gaps are present. The insert is thereby torn or otherwise damaged in the area of the swaged lip, and, more importantly, the sealing surface is depressed to such an extent that it fails to adequately seal against the valve member. Again, the defective inserts may be discovered during a shell test, but only after the seals have been installed in the valve body. The valve must then be disassembled and provided with new seats before it is reassembled and put in service. Consequently, even if a defect in the insert is discovered during the shell test, costly time is wasted and considerable difficulty is encountered in assembling and disassembling the valve.